Different techniques exist for measuring mechanical properties of elasticity of thin-film materials.
Thin-film elasticity measurements are commonly performed by indentation. Indentation consists in applying a determined load at the surface of a material then measuring the indent induced by the deformation of the material. Indentation is, by nature, destructive. Moreover, indentation involves simultaneously the elastic properties of compression and shearing, as well as the plasticity of the material. The quantitative analysis of the indent is hence complex. Finally, indentation does not allow quantifying the adhesion of a material.
The spontaneous Brillouin scattering technique is based on the inelastic scattering of an incident continuous light beam by incoherent phonons of thermal origin naturally present within the medium to be analysed. The scattered optical signal contains the information about the phonon velocity, which gives access to the elasticity of the medium and in particular to the anisotropic elasticity. The spontaneous Brillouin scattering technique has been applied in very numerous fields. However, the very low level of the Brillouin scattering signal generally requires a significant interaction volume. To overcome this limit, the publication of A. A. Stashkevich, P. Djemia, Y. K. Fetisov, N. Bizière and C. Fermon, “High-Intensity Brillouin light scattering by spin waves in a permalloy film under microwave resonance pumping”, J. Appl. Phys. 102, 103905, 2007, proposes an amplification of the signal from an external microwave source. Moreover, the use of phase-grating spectrometers has allowed increasing the sensitivity of detection of the Brillouin scattering photons. The publication of G. Scarcelli and S. H. Yun, “Confocal Brillouin microscopy for three-dimensional mechanical imaging”, Nat. Photonics 2, 39-43, 2008, illustrates experiments of confocal Brillouin microscopy for imaging mediums, in particular biological mediums. However, the very low amplitude of the incoherent thermal phonons requires, on the one hand, degrading the spatial resolution of the images and needs, on the other hand, a point-by-point acquisition, which makes impossible the real-time image acquisition.
The measurement technique based on the spontaneous Brillouin scattering is very little used outside laboratories. This technique is incompatible with the full-field imaging and does not allow deducing therefrom measurements of adhesion between materials.
In microelectronics, the metrology of the thin layer elasticity properties is performed by a picosecond acoustics technique. Picosecond acoustics is a time-resolved pump-probe technique that uses a pump light beam comprising one or several laser pulses for generating coherent acoustic phonons and a timely-offset probe light beam with an adjustable delay with respect to the pump beam for detecting the Brillouin scattering on the coherent phonons in a spectral range from GHz to THz. The pump laser pulses are ultra-short laser pulses. In the present document, it is meant by ultra-short pulse a pulse whose duration is comprised between about ten femtoseconds and about hundred picoseconds. The pump light beam and the probe light beam may come from a same source or from two distinct sources. The pump and probe light beams may have the same wavelength or different wavelengths.
FIG. 1 schematically shows a picosecond acoustic device and method according to the prior art. A sample 2 to be analysed has an interface 3 with an optoacoustic transducer 1. By way of example, the optoacoustic transducer 1 is metallic or includes a thin-film metallic coating. The sample may be deposited or fastened, for example by bonding, on the optoacoustic transducer 1. A pump light beam, or pump beam, 10 consisted of a single laser pulse of sub-picosecond duration is considered. The single-pulse pump light beam 10 is incident on the sample 2 to be analysed and transmitted towards the interface 3. The partial absorption of the pump laser pulse by the transducer material 1 generates, by optoacoustic conversion, an acoustic front 20 that propagates in the sample 2 in the opposite direction of the incident laser pulse. This acoustic front 20 generates a transitory deformation field that induces disturbances of the refractive index of the sample. A probe light beam, or probe beam, 15 is directed towards the sample. In the example of FIG. 1, the pump beam and the probe beam are in normal incidence on the sample. The probe beam 15 is timely offset with respect to the pump beam 10. A portion 11 of the probe beam that is not absorbed is reflected at the interface 3 between the optoacoustic transducer 1 and the sample 2. Another portion 12 of the probe beam is back-scattered due to the Brillouin interaction on the acoustic wave front 20. The reflected beam and the back-scattered beam interfere with each other and create temporal modulations or oscillations. The frequency of these interferences is linked to the velocity of the coherent phonons in the sample. The reflected beam and the back-scattered beam are collected. The detection of the time trace of the relative change of reflectivity of the probe beam allows measuring the disturbances of the refractive index of the sample. For a sample consisted of a propagation medium transparent enough to the pump beam and to the probe beam, the picosecond acoustics technique allows detecting in the time domain Brillouin oscillations that give access to the velocity of the acoustic waves. An advantage of this technique comes from the fact that the amplitude of the Brillouin scattering signal generated by the conventional picosecond acoustics technique, i.e. by coherent phonons, is higher than when this diffusion is initiated by thermal phonons.
However, in a conventional picosecond acoustics experiment, the pulse excitation generates a wideband spectrum of coherent phonons extending up to the terahertz (THz). Only the spectral component of the transitory acoustic signal in tune with the Brillouin frequency then contributes to the photo-elastic interaction. The detection of the coherent phonons by picosecond acoustics hence requires the use of a synchronous detection system. Now, to our knowledge, there exists no imaging system based, for example, on a CCD camera, having a synchronous detection on each pixel of the camera, so that full-field detection on a CCD camera is not possible in picosecond acoustics.
The developments of the picosecond acoustics technique have been initiated by researches in solid state physics and the main industrial applications are found in the field of microelectronics. First applications of the picosecond acoustics technique to biology have allowed access to the mechanical properties of a biological cell at a sub-cell scale, represented as an image.
The picosecond acoustics technique can also be applied to non-transparent thin layers. It may give access to thickness measurements. The very low signal-to-noise ratio makes it necessary to use a synchronous detection. However, the acquisition times are long, not easily compatible with imagery.
During measurements of Brillouin lines by the conventional picosecond acoustics technique, the spectral resolution is limited by the repetition frequency of the pulse laser, generally of the order of 80 MHz or 50 MHz.
The document H. J. Maris “Picosecond ultrasonics” Sci. Am. 278, 64-67, 1998, describes a measurement device based on the emission of a picosecond laser pulse incident on a multi-layer sample to be analysed. A first picosecond laser pulse, called pump pulse, heats the sample and induces an acoustic pulse propagating in the multi-layer sample. This acoustic pulse is transmitted through the different layers towards the surface of the sample and modifies the optical properties of the surface. A second picosecond laser pulse, called probe pulse, is directed to the sample. By comparing the temporal shape of the optical wave reflected on the surface with that of the probe pulse emitted, the instant at which the echo of the acoustic wave reaches a peak can be determined, with a sub-picosecond resolution. The picosecond ultrasonics technique hence allows non-destructively measuring the thicknesses and the interfaces of a multi-layer sample with accuracy lower than one Angstrom.
The publication H. N. Lin, R. J. Stoner, H. J. Maris and J. Tauc, “Phonon attenuation and velocity measurement in transparent materials by picosecond acoustic interferometry, J. Appl. Phys., 69, 3816, 1991”, describes another technique of picosecond acoustic interferometry, in which a picosecond pump pulse is transmitted through a sample to be analysed then absorbed on an interface with a metal film. This absorption generates coherent acoustic phonons that propagate in the sample and induce a local modification of the optical index. A picosecond probe pulse, timely offset with respect to the pump pulse, is directed to the sample. The probe pulse creates multiple scatterings and reflections between the phonons and the interface with the metal film. The measurement of the variations of optical reflectivity on the sample as a function of time makes appear oscillations due to the interferences between the multiple scatterings and reflections. The analysis of these picosecond acoustic interferometry measurements as a function of time allows deducing therefrom the velocity of the phonons.
The publication G. Scarcelli and S. H. Yun, “Confocal Brillouin microscopy for three-dimensional mechanical imaging”, Nat. Photonics 2, 39-43, 2008, describes a confocal microscopy device based on a continuous laser source and a Brillouin scattering spectrometer to form two- or three-dimensional (2D or 3D) optical microscopy images, revealing the mechanical properties of a biological cell in solution. However, the intensity of the Brillouin scattering spectra is generally very low.
The publication A. A. Stashkevich, P. Djemia, Y. K. Fetisov, N. Bizière and C. Fermon, “High-Intensity Brillouin light scattering by spin waves in a permalloy film under microwave resonance pumping”, J. Appl. Phys. 102, 103905, 2007, describes a Brillouin scattering measuring device assisted by a microwave excitation. This coupling of a light excitation source and a microwave excitation makes it possible, according to the authors, to increase the Brillouin scattering measurement by three orders of magnitude when there is a resonance with spin waves.
The publication T. Dehoux, M. Abi Ghanem, O. F. Zouani, J.-M. Rampnoux, Y. Guillet, S. Dilhaire, M.-C. Durrieu and B. Audoin, “All-optical broadband ultrasonography of single cells”, Scientific Reports, 5:8650, DOI 10.1038, 2015, describes an optoacoustic pulse inverted microscope comprising a first and a second picosecond pulse laser, a photodiode detector and an optoacoustic transducer consisted of a sapphire substrate having an interface with a titanium film. The first laser emits pump laser pulses at a first wavelength and a first repetition frequency towards the sapphire substrate of the optoacoustic transducer, the other interface of the titanium film being in contact with a single-cell to be analysed. Each pump laser pulse is absorbed into the titanium film and generates an acoustic pulse, which is reflected on the titanium-cell interface. The second laser emits probe pulses at another wavelength and another repetition frequency. The probe pulses are focused to the titanium-sapphire interface, hence allowing detecting the mechanical properties of the observed cell. The photodiode acquires, as a function of time, the optical reflectivity variations by elasto-optical coupling on the titanium-sapphire interface. This optoacoustic pulse inverted microscope allows studying the adhesion of a single cell to a titanium film. In this system, no laser pulse reaches the titanium-cell interface. The drawback of this system is that the acquisition, made point-by-point, is long.
It is desirable to have at one's disposal a fast analysis of the acoustic properties of inhomogeneous materials, thin layers or biological cells, which is two dimensionally-, or possibly 3D-, resolved over distances of several hundreds of nanometres to several microns, while having an increased sensitivity.